The present invention relates to a magnetic recording medium such as magnetic tape, magnetic sheet and magnetic disk.
The use of finer magnetic particles in magnetic recording media, especially those for video and computer applications, has increased today in order to meet the demands for higher recording density and S/N ratio.
It is generally held that the S/N ratio of magnetic recording media (hereinafter sometimes referred to as "magnetic tape") is proportional to the square root of the number of magnetic particles in the recording material that are related to recording and reproduction. Thus, with the same weight of magnetic powder being applied, the S/N ratio can be effectively improved by using finer magnetic particles. However, as magnetic particles become finer, their surface activity will increase to make it difficult to disperse them uniformly. While various dispersants have conventionally been used to improve the dispersability of magnetic particles, they suffer the disadvantages that problems such as blooming and surface tack will arise if they are added in sufficient amounts to insure satisfactory dispersability.
If the size of magnetic particles is reduced to provide a higher BET value, the surface of the magnetic layer becomes smooth by a corresponding amount and the tape's endurance to repeated running decreases rather than increases. Magnetic tape makes vigorous contact with the magnetic head during recording and reproduction cycles and if it has an extremely smooth surface, the contact area and hence the frictional coefficient will increase to impair its running property. During repeated use of the tape, this can cause such consequent problems as the wear of the magnetic layer, shedding of the magnetic particles in said layer, and clogging of the magnetic head due to the shed particles.
If an even higher S/N ratio is required as in video tapes, not only are the magnetic particles refined but also nonmagnetic fillers such as carbon black and abrasives are removed from the magnetic layer to increase the loading density of magnetic particles. However, if carbon black is removed from the magnetic layer, the optical density of the magnetic tape is reduced to introduce difficulty in controlling tape running in such a way that it can be run or stopped at a desired time. Furthermore, electroconductivity is lost and static buildup will impair the tape durability to repeated running.
Further, when magnetic tape is caused to run on a video deck, not only its obverse surface but also its reverse surface undergoes abrasion by guide poles and rollers. Therefore, if the running property and the durability of the back surface of the tape are not good, an irregular and sudden tension will be exerted upon the running tape to cause excessive abrasion of the magnetic layer by the head. As a result, not only is the magnetic layer damaged but also the shedding of magnetic particles will occur. In addition, the tension at which the tape is wound varies constantly, causing unevenness in winding pressure and the form in which tape is wound up. As a result, the edges of turns of wound tape become nonuniform and this results in uneven running in subsequent use of the tape. All these phenomena contribute to deterioration of image and the electromagnetic characteristics of the tape as typified by skew, jitter and low S/N ratio.
With a view to overcoming these problems, it has been proposed that a back coat (hereinafter sometimes referred to as a "BC layer") be provided on the reverse side of magnetic tape. An example of such proposal is the inclusion and dispersing of an inorganic powder in a binder. This approach is intended to roughen the surface of the BC layer so that the area of contact with guide poles and rollers is sufficiently reduced to realize a lower coefficient of friction. See, for example, JP-A-57-130234 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-57-53825, JP-A-58-161135 and JP-A-58-2415.
The use of carbon black in place of inorganic powders has also been proposed as in JP-B-52-17401 (the term "JP-B" as used herein means an "examined Japanese patent publication"). The technique disclosed in this publication aims at preventing static buildup by the electroconductivity of carbon black, as well as the light-shielding and surface-roughening effects of its particles. However, the carbon black particles used have very low dispersibility in paints or coating solutions. When a BC layer is formed by applying the resulting dispersion, the carbon black particles will agglomerate and produce coarse asperities on the surface. Further, such agglomerated particles adhere so weakly to the binder that they will easily shed off the surface. In addition, when the tape is wound and if adjacent turns contact each other, large agglomerated particles in the back coat of one layer will produce asperities on the magnetic coating of the adjacent layer.
With a view to avoiding the aforementioned problems with the obverse and reverse faces of magnetic tapes, it has been proposed in JP-A-58-164020, JP-A-61-276120 and JP-A-62-188018 that an undercoat of a carbon black containing resin be provided. However, the dispersability of carbon black in the undercoat is still too poor to avoid the formation of asperities, which will consequently produce asperities in the magnetic layer to induce a lower S/N ratio. Further, the adhesion between the base support, undercoat and magnetic layer is so weak as to induce separation of these layers.
In order to disperse magnetic particles and fillers, it is necessary to perform prolonged mixing and dispersing operations and to use dispersants in large quantities. But this has inevitably impaired the characteristics of tape and deterioration of the surface properties and it has thus been difficult to improve dispersability by a desired degree.
Under these circumstances, modified binder resins that have anionic functional groups introduced therein to assist in dispersing magnetic particles and carbon black have been studied and they have proved to be effective in some way. However, even such modified resins are still unsatisfactory in their ability to disperse magnetic particles, in particular carbon black particles, having a high BET value.